Monovinylidene aromatic polymers such as polystyrene and rubber-modified polystyrene are very easily thermoformed and exhibit impact strength and low temperature properties when thermoformed. Thermoforming is a process whereby a resin sheet or preform which is heat softened to a temperature below the temperature at which the resin is completely molten or plastified, is formed into the desired shape by an applied pressure or vacuum. However, monovinylidene aromatic polymers with a high degree of crystallinity are brittle and can be thermoformed only at high temperatures. Monovinylidene aromatic polymers are known to have poor environmental stress crack resistance. Environmental stress cracking occurs when molded resin parts are subjected to conditions where the part is both placed under stress and exposed to an oil- or grease-containing substance. These conditions occur commonly where materials such as grease- or oil-containing foods are packaged in a molded container. The presence of the grease or fat in the food in combination with the stress placed on the container, associated with filling, sealing and handling, cause such containers to become weakened and easily damaged.
Olefinic polymers, such as polyethylene, polypropylene, and the like have relatively good resistance to the actions of oils and greases and, therefore, are very desirable for the manufacture of containers for foods and similar materials. When it comes to manufacturing or forming techniques, however, olefinic polymers are much less versatile than monovinylidene aromatic polymers. Olefinic polymers are very difficult to thermoform due to their low glass transition temperature and relatively sharp melting point at elevated temperature.
It is desirable to combine the toughness and solvent resistance of olefinic polymers with the high modulus and thermoformability of monovinylidene aromatic polymers by blending the two resins. Blending of monovinylidene aromatic and olefinic polymer resins is, however, complicated by the incompatibility of the two resins. Attempts to solve the incompatibility problem have focused on the use of specified amounts of the two resins and also on the use of a so-called compatibilizer.
Compatibilizers for blends of olefinic polymers and monovinylidene aromatic polymers proposed heretofore include:
graft copolymers of polystyrene and polyethylene, see for example Polymer 18, 69 (1977), J. Appl. Polym. Sci. 17, 2597 (1973), Polymer 21, 1469 (1980), U.S. Pat. Nos. 4,690,976, and 4,716,197;
graft copolymers of polystyrene and EPDM rubber, see for example J. Appl. Polym. Sci. 17, 2597 (1973), ANTEC '92 Conference Proceedings, Society of Plastic Engineers, May 3-7, Detroit 2635 (1992);
di-block copolymers of styrene-diene, see for example JP-A-48-43,031, JP-A-49-28,637, U.S. Pat. No. 4,031,166, EP-A-0,060,524, and EP-A-0,060,525;
hydrogenated di-block copolymers of styrene-diene, see for example Polymer Blends, E. Martuscelli, Ed. 201 (1981), J. Polym. Sci.: Polym. Letters, Ed. 19, 79 (1981), J. Polym. Sci.: Polym. Phys. Ed. 19, 1269 (1981), and J. Polym. Sci.: Polym. Phys. Ed. 20, 2209 (1982), GB-A-1,363,466, DD-A-241,375, U.S. Pat. No. 4,020,025, and JP-A-81-38,338;
block copolymers of styrene-ethylene, see for example U.S. Pat. Nos. 3,980,736 and 3,851,015;
tri-block copolymers of styrene-diene-styrene, see for example Eur. Polym. J. 19, 81 (1983), and U.S. Pat. Nos. 4,386,187 and 4,386,188;
hydrogenated tri-block copolymers of styrene-diene-styrene, see for example ANTEC '92 Conference Proceedings, ibid., EP-A-0,060,525, U.S. Pat. No. 4,188,432, Polym. Eng. & Sci. 21(5), 985 (1981), J. Appl. Polym. Sci. 26, 1 (1981), and Polymer Blends and Alloys Technology, Vol. 3, Technomic Publishing Co., 117 (1992); and
an elastomer comprising styrene and a diene hydrocarbon monomers on the main chain of polymer, which monomers are distributed heterogeneously microscopically but homogeneously macroscopically. Such elastomer can be obtained by polymerizing styrene and a diene in the presence of a lithium based catalyst to produce copolymer which is then further added to a mixture of the monomers and continued to be copolymerized, see JP-56-36,534.
It is commonly hypothesized that compatibilization between two immiscible or incompatible polymers can be effected by a compatibilizer which improves adhesion between the polymer interfaces. It is further hypothesized, that for good adhesion the compatibilizer must have molecular segments which are miscible with the respective polymer domains. This would require the compatibilizer to have certain blocks of structures compatible with the respective polymer domains. For this reason, the compatibilizers proposed heretofore generally are grafted copolymers or block copolymers. The block copolymers containing unsaturated bonds in the polymer backbone lack resistance to thermal oxidation and ultraviolet light, presenting difficulties during processing and while the article is in use. Adding large amounts of antioxidant reduces the processing problem but will lead to the antioxidant migrating to the surface of fabricated articles, thereby deteriorating the aesthetics of the article. Saturated block copolymers having hydrogenated rubber blocks such as styrene-ethylene/butylene-styrene (SEBS) tri-block copolymers are more stable but in general not as effective compatibilizers as unsaturated ones. In addition, block copolymers and especially the hydrogenated block copolymers are expensive due to their high cost manufacturing process. Accordingly, there is still a desire for other low-cost compatibilizers which perform at least equally to or even better than existing compatibilizers. In addition there is a desire for compatibilizers which are resistant to oxidation and ultraviolet light, using decreased amounts of or no antioxidants.
There is also a desire for foamable or expandable materials which combine the properties of olefinic polymers and monovinylidene polymers in an expanded or foamed application.